1. Field of the Invention
The present invention relates to a pneumatic tire, and more particularly, to a pneumatic tire used as an excellent studless tire which shows little variation in startup/braking ability and controllability on icy and snowy road surfaces between initial and final periods of tire use.
2. Description of Related Art
In a conventional pneumatic tire, the area of the tire contacting with icy or snowy road surfaces is increased by decreasing the hardness of the tread rubber for attaining startup/braking ability and controllability of the pneumatic tire on icy or snowy road surfaces. More specifically, a polymer having a low glass transition temperature is used in the tread rubber composition, a softener is added to the tread rubber composition, or a foamed rubber is used in the tread. When a foamed rubber is used as the tread rubber, it is found that sufficient performance on icy or snowy road surfaces cannot always be exhibited in the final period of tire use because the tread rubber composition is hardened during driving, and the area contacting with icy or snowy road surfaces is decreased, although the decrease is less significant than that of a tread in which a foamed rubber is not used. It is disclosed in Japanese Patent Application Laid-Open No. 58-87138 that, in a conventional tire, a rubber composition containing a specific vulcanization accelerator and a specific antioxidant shows improvement in hardening under heating, and in the appearance of a tire in the final period of use. However, nothing is mentioned about a tire having a foamed tread and the performance of a tire having a foamed tread on ice or snow.
The main factor related to hardening has been found to be the additional crosslinking reaction caused by sulfur and vulcanization accelerators which remain in the rubber. In conventional technology, suppressing hardening caused by the additional crosslinking reaction in a foamed tread of a tire is considered to be difficult, and no means for suppressing deterioration in performance in the final period of tire use has been known.
Because such a deterioration in performance takes place, performance on ice or snow in the final period of tire use has heretofore been maintained by enhancing performance in the initial period still more to compensate for the deterioration. For this purpose, a large amount of a polymer having a low glass transition temperature, such as polybutadiene, is used for lowering hardness in the initial period. However, this method causes a formation of uneven wear in the final period of tire use and a decrease in the area in contact with icy and snowy surfaces, and this becomes another reason for deterioration in performance on ice or snow in the final period of tire use.